This invention concerns a method for the continuous rolling of plate and/or strip, and the relative continuous rolling line.
To be more exact, the invention arranges to produce plate and/or strip, starting from at least two continuous casting lines or else from one continuous casting machine with two lines for the production of thin and medium-sized slabs of steel, where the slab as it enters the roughing train is between 60 and 120 mm thick, and the lines are positioned side by side or cooperate with each other and are associated with the same roughing train and the same finishing rolling train.
The rolling line according to the invention is pre-arranged to process continuously, that is to say, to provide a substantially continuous feed of slab to the finishing train throughout the whole casting cycle of all the casting lines working in cooperation with the finishing train.
The thin and medium-sized slabs to which the invention is applied have a thickness between 60 mm and 120 mm, advantageously between 70 and 90 mm when they enter the roughing train. In other words, the invention arranges to obtain, at the outlet of the crystallizer, pre-slabs with a substantially uniform thickness. The invention also teaches to obtain slabs with a thickness coherent with the requirements of the rolling line, by means of a process of soft-reduction applied to the pre-slab immediately after the crystallizer.
The rolling line according to the invention is suitable to produce plate and/or strip having a minimum finished thickness of about 0.8 mm to 1.5 mm.
Stahl und Eisen, Vol. 108, n.degree.. 3 pages 99-109 describes lines for the rolling of plate and/or strip which include one or more continuous casting machines for slabs of a fixed thickness of 50 mm that tend-the same one finishing train. This fact alone makes the system described therein extremely rigid.
This finishing train may be positioned on the same axis as one of the casting lines or may be in an intermediate position between the two casting lines.
Such lines normally include transfer systems, which consist of heating furnace systems or at least of temperature-maintaining furnace systems and which transfer the slab from a casting line or lines which are located in a position offset from the finishing train.
In such lines, the furnaces are used to heat the slabs from the end-of-casting temperature to the optimum temperature for rolling.
Another important function of such furnaces is to create a buffer stock of sufficient size to keep the continuous casting working even during interruptions in the rolling process, for example when rolls have to be changed.
This lay-out entails interruptions in the feed to the finishing train between the end of the processing of one slab and the beginning of the processing of the next slab.
The interruptions are caused by the fact that, with current casting speeds, in the event of two casting machines or one casting machine with two lines, it is not possible to obtain a sufficient production when the slab is 50 mm thick.
These interruptions lead to a discontinuous working of the rolling train with transient moments of intake of a slab, these moments causing disturbances of the system and entailing unfavourable effects on the thickness, width, profile and planar condition of the strip.
Furthermore, these interruptions involve the risk of failure to feed the rolled product into the rolling mill and into the coiling reel and therefore of jamming with a resulting loss of production and damage and wear to the rolling rolls.
In other words the rolling mill works constantly in a transient condition with the above unfavourable results.
In fact, when the thin slab has a starting thickness of 50 mm, in order to obtain a 2.5 mm strip the speed of the finishing train must be 6.4 meters per second, which corresponds to a value of steel flow per unit of strip width of 960, the product of the speed in m/min multiplied by the thickness in mm.
The value of between 800 mn.m/min and 1100 mn.m/min must be respected, and cannot be reduced, if the correct end-of-rolling temperature (between 840 and 880.degree. C.) is to be obtained. When the slab is 50 mm thick, two casting lines should cast at the speed of 9 meters per minute, which for the moment is an unattainable objective, as the maximum casting speed which can be achieved at present is around 6 meters per minute for that thickness of slab.
Therefore, these interruptions of the feed prevent the best exploitation of the potential of the finishing train, which is forced to work in a discontinuous manner and thus restricts the quality and overall output of the plant, particularly when slender thicknesses such as those less than 1.2 mm are being produced.
Indeed, it is a serious problem in the rolling of thin strip, as the rolling speeds cannot increase beyond a certain limit since they are restricted by the problems of the feed of the leading end of the strip onto the roller conveyor at the outlet from the finishing train; in fact, as the leading end of the thin strip is fed onto the roller conveyor as it leaves the finishing train, it tends to be raised on the roller conveyor and to bounce backwards.